1. Field of the Invention
The invention relates to a method for digital transmission of hierarchical HDTV, EDTV and SDTV television signals (HDTV=High Definition TV, EDTV=Extended Definition TV and SDTV=Standard Definition TV).
2. Description of the Prior Art
Multiresolution modulation is prior art for the transmission of digital hierarchical television signals (see K. K. Uz, K. Ramchandran and M. Vetterli, "Multiresolution source and channel coding for digital broadcast of HDTV" Fourth International Workshop on HDTV and beyond, Torino, September 1991; T. Cover, "Broadcast Channel", IEEE Trans. on IT, IT-18, pages 2 to 14, January 1972). For error protection in the transmission various channel coding methods are known, such as convolutional codes, block codes, and so on. In the combination of channel coding with modulation trellis-coded modulation (TCM) (cf. G. Ungerboeck, "Channel Coding with multilevel phase signals", IEEE trans. IT, Vol. IT-28, pages 55 to 67, January 1982), block-coded modulation (BCM) (cf. S. I. Sayegh, "A class of optimum block codes in signal space", IEEE Trans. on Com., COM-34, pages 1043 to 1045, October 1986) and multilevel coding (cf. A. R. Calderbank, "Multilevel codes and multistage decoding" IEEE Trans. on Com, COM-37, pages 222 to 229, March 1989) are known. For a more efficient transmission via a bandwidth limited channel, orthogonal frequency division (OFD) multiplexing is also used (cf. S. B. Weinstein and P. M. Ebert, "Data transmission by FDM using discrete Fourier Transform", IEEE Trans. on Com., COM-19, pages 628 to 634, October 1971).
With the means known at present transmission of digital hierarchical television signals is not possible in a 7 MHz channel when the following demands are to be satisfied.
(a) An extremely high total data rate is to be transmitted substantially free of error via the channel available.
(b) The differently weighted data are to be protected for channel errors in accordance with their priority.
(c) A modulation method is to be found in which the respective receivers can extract the required information from the data stream transmitted.
(d) Furthermore, a transmission method is to be found which permits different receiver compatible complexity corresponding to the quality of the received television signal.
(e) Furthermore, the method is to be devised so that error concealment and graceful degradation can be carried out as far as possible.
(f) The transmission method is to be devised so that a stationary, portable and mobile reception is possible with different respective transmission channels.
A hierarchical transmission of HDTV, EDTV and SDTV signals can be carried out with the multiresolution modulation without multilevel coding, but only when a considerably larger bandwidth is available for the transmission. Likewise, the transmission could be carried out with the known TCM and BCM methods, although only with the same very high receiver complexity and without graceful degradation.
Various other attempts have been made to meet some of the demands enumerated above under (a) to (g). As modulation method, at least a 64 quadrature amplitude modulation (QAM) should be used. In addition, channel coding is necessary to ensure that for a given symbol energy/ one side noise power (Es/No) and the particular channel properties the signal can be received with the desired bit error probability. It has further been suggested to transmit the various EDTV signals in a common signal by multiresolution modulation. This also made graceful degradation possible so as to obtain only a somewhat poorer television quality instead of total loss when the receiving conditions become worse.